beyond American knowledge, and houses built upon soil
saturated for centuries with the offal of successive and uncleanly
generations. Wet earth, kept wet by the boiling up of imprisoned
springs, is a focus of disease. Dry earth is one of the most perfect
deodorizers, the best of oxydizers and absorbents, destroying the germs
of disease with wonderful certainty. On those two facts rests the theory
of public hygiene."

DUST AND DISEASE.

The air we breathe is heavily loaded with minute particles of floating
dust, their presence being revealed only by intense local illumination.
Professor Tyndall says: "solar light, in passing through a dark room,
reveals its track by illuminating the dust floating in the air. 'The
sun,' says Daniel Culverwell, 'discovers atoms, though they be invisible
by candle-light, and makes them dance naked in his beams.'"

After giving the details and results of a series of experiments in which
he attempted to extract the dust from the air of the Royal Institute by
passing it through a tube containing fragments of glass wetted with
concentrated sulphuric acid, and thence through a second tube containing
fragments of marble wetted with a strong solution of caustic potash,
which experiments were attended with perfect failure, the Professor
continues, "I tried to intercept this floating matter in various ways;
and on the day just mentioned, prior to sending the air through the
drying apparatus, I carefully permitted it to pass over the tip of a
spirit-lamp flame. The floating matter no longer appeared, having been
burnt up by the flame. It was, therefore, of _organic origin_. I was by
no means prepared for this result; for I had thought that the dust of
our air was, in great part, inorganic and non-combustile." In a foot
note he says, "according to an analysis kindly furnished me by Dr.
Percy, the dust collected _from the walls_ of the British Museum
contains fully fifty per cent of inorganic matter. I have every
confidence in the results of this distinguished chemist; they show that
the _floating_ dust of our rooms is, as it were, winnowed from the
heavier matter." Again he says: "the air of our London rooms is loaded
with this organic dust, nor is the country air free from its presence.
However ordinary daylight may permit it to disguise itself, a
sufficiently powerful beam causes dust suspended in air to appear almost
as a semi-solid. Nobody could, in the first instance, without
repugnance, p